WASHINGTON — The Senate Foreign Relations Committee today unanimously approved President Barack Obama’s choice of Sen. John Kerry to succeed Hillary Rodham Clinton as secretary of state. By voice vote, the panel approved the nomination of the five-term Massachusetts Democrat, who has been a member of the committee for 28 years and led it for the past four. The full Senate planned to vote this afternoon. Kerry did not attend the session in the ornate diplomatic room in the Capitol. In his absence, Democrats and Republicans praised him and remarked on his extensive grasp of the issues during his confirmation hearing last Thursday. Sen. Bob Menendez, D-N.J., who succeeds Kerry as committee chairman, said the senator would be a “formidable secretary of state.” Obama chose Kerry, 69, the son of a diplomat, decorated Vietnam veteran and 2004 Democratic presidential candidate, to succeed Clinton, who is stepping down after four years. The senator had pined for the top diplomatic job that went to Clinton after Obama’s 2008 election. Kerry has served as Obama’s unofficial envoy, soothing relations with leaders in Afghanistan and Pakistan.